Gravity Gradiometer

ABSTRACT

The present invention provides a gravity gradiometer for measuring components of the gravity gradient tensor. The gravity gradiometer comprises a housing and a gimballed support structure with at least one actuator. The gravity gradiometer further comprises at least one sensor mass arranged for movement in response to a gravity gradient. The gravity gradiometer also comprises a transducer that is arranged so that the movement of the at least one sensor mass relative to a portion of the transducer generates a transducer signal. Further, the gravity gradiometer comprises an electrical feedback loop arranged to direct an electrical signal to the actuator of the external support structure. The gravity gradiometer is arranged so that an influence of an external angular acceleration on the rotation of the housing is reduced by the actuator when the transducer generates a transducer signal effected by the external angular acceleration on the gravity gradiometer.

FIELD OF THE INVENTION

The present invention relates to a gravity gradiometer and to componentsfor high precision measurement instruments.

BACKGROUND OF THE INVENTION

Gravimeters are used in geological exploration to measure the firstderivatives of the earth's gravitational field. Whilst some advanceshave been made in developing gravimeters which can measure the firstderivatives of the earth's gravitational field because of the difficultyin distinguishing spatial variations of the field from temporalfluctuations of accelerations of a moving vehicle, these measurementscan usually be made to sufficient precision for useful exploration onlywith land-based stationary instruments.

Gravity gradiometers (as distinct from gravimeters) are used to measurethe second derivative of the gravitational field and use a sensor whichis required to measure the differences between gravitational forces downto one part in 10¹² of normal gravity.

Typically such devices have been used to attempt to locate deposits suchas ore deposits including iron ore and geological structures bearinghydrocarbons.

The gravity gradiometer typically has at least one sensor in the form ofsensor mass which is pivotally mounted for movement in response to thegravity gradient.

International publication WO 90/07131, partly owned by the presentapplicant's associated company, discloses such a gravity gradiometer.Gravity gradiometers of that type are typically mounted in an aircraftand carried by the aircraft while making measurements. The consequenceof this is that the gravity gradiometer can move with movements of theaeroplane. This creates accelerations of the gradiometer which aredetected by the gravity gradiometer and if not compensated for, willproduce noise or swamp actual accelerations or movement of thegradiometer in response to the gravity gradient which is to be detectedby the gravity gradiometer.

The gravity gradiometer disclosed in International publication WO90/07131 includes two sensor masses which are orthogonally positionedand arranged to move about a common axis. The sensor masses aresuspended by pivots and can oscillate in planes that are orthogonal tothe common axis. For measurement of the gravity gradient the instrumentis continuously rotated and a local change in the gravity gradientresults in oscillating of both sensor masses relative to a rotatedhousing of the instrument. Such arrangement has the advantage that atleast some unwanted accelerations, such as those resulting from a suddenmovement of an aircraft, are experienced by both sensor masses in thesame manner and can be eliminated.

The present invention provides technological advancement.

SUMMARY OF THE INVENTION

The present invention provides a gravity gradiometer for measuringcomponents of the gravity gradient tensor, the gravity gradiometercomprising:

-   -   a housing;    -   a gimballed support structure for supporting the housing and        rotating the housing about an axis, the support structure        comprising an actuator;    -   at least one sensor mass being positioned in the housing and        being arranged for movement in response to a gravity gradient;    -   a pivotal coupling enabling the movement of the at least one        sensor mass about an axis and suspending the at least one mass        in the housing;    -   a transducer arranged so that the movement of the at least one        sensor mass relative to a portion of the transducer generates a        transducer signal; and    -   an electrical feedback loop arranged to direct an electrical        signal to the actuator of the external support structure, the        electrical signal being associated with the transducer signal;    -   wherein the gravity gradiometer is arranged so that an influence        of an external angular acceleration on the rotation of the        housing is reduced by the actuator when the transducer generates        a transducer signal effected by the external angular        acceleration on the gravity gradiometer.

The gravity gradiometer may be arranged for movement over a ground planethat defines an x-y plane of an x,y,z Cartesian coordination system.

The support structure typically is arranged so that in use the housingis rotated about an axis oriented substantially along the z-direction ofthe x,y,z Cartesian coordination system.

In one specific embodiment of the present invention the gimballedsupport structure is triaxial and comprises three substantiallyorthogonal axes. In this embodiment the gimballed support structurecomprises at least two additional actuators arranged for rotating thehousing about each of the three substantially orthogonal axes. Further,the gimballed support structure may comprise gyroscopes that detectrotation about respective axes and that are coupled to respectiveactuators in a manner such that the actuators correct for rotationgenerated by the external angular acceleration. In this embodiment theelectrical feedback loop is arranged so that the electrical signal,being associated with the transducer signal, is used to further reducethe influence of the external angular acceleration. In one specificembodiment the electrical feedback loop is arranged so that rotation ofthe housing about the z-axis is fine-tuned and influence of the externalangular acceleration on the rotation about the z-axis is substantiallyavoided.

The transducer may comprise a capacitor, such as a constant chargecapacitor.

The pivotal coupling may comprise a flexure web for connecting the atleast one mass in a housing for movement in response to the gravitygradient.

The flexure web may be integral with the at least one sensor mass andhousing to form a monolithic structure. However, the flexure web mayalso be formed on a separate flexure web element and connected to thehousing and the at least one sensor mass.

The at least one sensor mass may be provided in any shape, but typicallyis a chevron shaped bar.

In one specific embodiment the gravity gradiometer comprises a pair oftransversally arranged sensor masses for movement in response to thegravity gradient. In this embodiment the transducer is one of aplurality of transducers and each sensor mass may be associated with twoor more transducers arranged so that movement of the sensor massesgenerates transducer signals that are associated with the electricalsignal used in the electrical feedback loop.

The invention will be more fully understood from the followingdescription of specific embodiments of the invention. The description isprovided with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a gravity gradiometer according to aspecific embodiment of the present invention.

FIG. 2 is a perspective view of a first mount forming part of a mountingof the gravity gradiometer of according to the specific embodiment ofthe present invention;

FIG. 3 is a perspective view of a second mount of the mounting accordingto a specific embodiment of the present invention;

FIG. 4 is a perspective view from underneath the mount shown in FIG. 3;

FIG. 5 is a view of the assembled structure;

FIG. 6 is a perspective view showing assembled components of the gravitygradiometer according to another specific embodiment of the presentinvention;

FIG. 7 is a plan view of a bar according to a specific embodiment of thepresent invention;

FIG. 8 is a diagram showing actuator control according to a specificembodiment of the present invention;

FIG. 9 is a perspective view of components of a gravity gradiometeraccording to a specific embodiment of the present invention;

FIG. 10 is a perspective view of a first mount of a mounting accordingto another specific embodiment of the present invention;

FIG. 11 is a perspective view of part of the mounting of FIG. 10 toillustrate the location and extent of the flexural web of the firstmount;

FIG. 12 is a perspective view of the mounting of FIG. 10 from beneath;

FIG. 13 is a perspective view of the mounting of FIG. 10 including asecond mount of the second embodiment;

FIG. 14 is a perspective view of a second mount component;

FIG. 15 is a perspective view of the second mount component of FIG. 14from above;

FIG. 16 is a perspective view of assembled components of the gravitygradiometer according to a specific embodiment of the present invention;

FIG. 17 is a plan view of a housing portion for supporting a baraccording to a further embodiment of the invention;

FIG. 18 shows a component of the gravity gradiometer according to anembodiment of the present invention;

FIG. 19( a)-(f) is a view of transducer elements according to a specificembodiment of the present invention;

FIG. 20 is a view similar to FIG. 18 but showing one of the transducerselements of FIG. 19 in place;

FIG. 21 is a diagram to assist explanation of the circuits of FIG. 22;

FIG. 22 is a circuit diagram relating to a specific embodiment of theinvention;

FIG. 23 is a frequency tuning circuit according to an embodiment of thepresent invention;

FIGS. 24 to 26 show circuitry according to embodiments of the presentinvention;

FIG. 27 is a cross-sectional perspective view through an actuatoraccording to a specific embodiment of the invention;

FIGS. 28( a) and (b) shows components of the gravity gradiometeraccording to a specific embodiment of the present invention; and

FIGS. 29 and 30 show block diagrams illustrating the operation of arotatable support system according to a specific embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

FIG. 1 is a schematic view of a gravity gradiometer 1 according to aspecific embodiment of the present invention. The gravity gradiometer 1is arranged for vertical positioning relative to a ground plane.Throughout this specification the ground plane coincides with an x-yplane of an x,y,z-coordination system and consequently the gravitygradiometer is in this embodiment arranged for orientation along thez-axis so that the Γ_(xy) and (Γ_(xx)-Γ_(yy)) components of the gravitygradient tensor can be measured.

The function of the gravity gradiometer 1 may be briefly summarized asfollows. The gravity gradiometer has in this embodiment twosubstantially identical sensor masses which are pivotally mounted on amounting so that they can oscillate relative to the mounting. The sensormasses with mounting are rotated about the z-axis and with an angularfrequency that approximately equals half the resonance frequency ofsensor masses. A gravity gradient will result in a force on the sensormasses which will then oscillate relative to the mounting during thatrotation. Components of the gravity gradient tensor can be determinedfrom the oscillating movement of the sensor masses. For further detailson the general principal of such measurements are described in theapplicants co-pending PCT international patent application numberPCT/AU2006/001269.

The gravity gradiometer shown in FIG. 1 comprises a housing 2 which isconnected to mount 3 for connection to an external platform (not shown).The external platform is arranged for rotation of the housing 2 at asuitable angular frequency about the z-axis. Further, the externalplatform is arranged for adjusting the housing 2 about three orthogonalaxes.

With reference to FIG. 2 a first mount 10 is now described. The firstmount 10 forms a part of rotatable mounting 5 which is shown in FIG. 5.The mount 10 comprises a base 12 and an upstanding peripheral wall 14.The peripheral wall 14 has a plurality of cut-outs 16. The base 12supports a hub 18.

FIGS. 3 and 4 show a second mount 20 which comprises a peripheral wall22 and a top wall 24. The peripheral wall 22 has four lugs 13 forsupporting the mounting 5 in the housing 2. The top wall 24 and theperipheral wall 22 define an 5 opening 28. The second mount 20 ismounted on the first mount 10 by locating the hub 18 into the opening 28and the lugs 13 through respective cut-outs 16 as is shown in FIG. 5.

The first mount 10 is joined to the second mount 20. The flexure web 31is formed in the first mount 10 so that a primary mount portion of themount 10 can pivot about a flexure web 31 relative to a secondary mountportion of the mount 10. This will be described in more detail withreference to the second embodiment shown in FIGS. 10 to 16.

The mounting 5 mounts the sensor 40 (which will be described in moredetail hereinafter and which is typically in the form of a massquadruple) for fine rotational adjustment about the z-axis forstabilizing the gradiometer during the taking of measurementsparticularly when the gradiometer is airborne. As described above,rotational stabilization about the x-and y-axis is provided by theexternal platform.

FIG. 6 shows sensor 40 mounted on the mounting. The sensor 40 is anOrthogonal Quadruple Responder—OQR sensor formed of a first mass and asecond mass in the form of a first bar 41 and a second bar 42 (not shownin FIG. 6) orthogonal to the bar 41 and which is of the same shape asthe bar 41.

The bar 41 is formed in a first housing portion 45 and the bar 42 isformed in a second housing portion 47. The bar 41 and the second housingportion 45 is the same as bar 42 and the second housing portion 47except that one is rotated 90° with respect to the other so that thebars are orthogonal. Hence only the first housing portion 45 will bedescribed.

The first housing portion 45 has an end wall 51 and a peripheral sidewall 52 a. The end wall 51 is connected to rim 75 (FIGS. 2 and 5) of thewall 14 of the first mount 10 by screws or the like (not shown). The bar41 is formed by a cut 57 in the wall 51 except for a second flexure web59 which joins the bar 41 to the wall 51. The second flexure 59 web isshown enlarged in the top view of the bar 41 in FIG. 7. Thus, the bar 41is able to pivot relative to the first housing portion 45 in response tochanges in the gravitational field. The bar 42 is mounted in the sameway as mentioned above and also can pivot relative to the second housingportion 47 in response to changes in the gravitational field about athird flexure web. The second housing portion 47 is connected to base 12(FIG. 2) of the first mount 10.

The bar 41 and the first housing portion 45 together with the secondflexure web 59 are an integral monolithic structure.

Transducers 71 (not shown in FIGS. 2 to 4) are provided for measuringthe movement of the bars and for producing output signals indicative ofthe amount of movement and therefore of the measurement of thedifferences in the gravitational field sensed by the bars.

FIG. 8 is a schematic block diagram showing actuator control tostabilize the gradiometer by rotating the mounting 5 about the z-axis. Acontroller 50 which may be a computer, microprocessor or the likeoutputs signals to actuators 53 and 54, which are arranged to rotate themounting 5 about the z-axis. Each actuator is positioned stationaryrelative to lugs 13 and coupled to the first mount 10 so that theactuator can effect a rotation by a small angle of the mount 10 withother components relative to the lugs 13 (and other components that arestationary relative to the lugs 13). Each actuator provides a linearmovement and is positioned so that the linear movement is translatedinto a small rotation of the mount 10. The actuators will be describedin more detail with reference to FIG. 27. The position of the mounting 5is monitored so that appropriate feedback can be provided to thecontroller 50 and the appropriate control signals provided to theactuators to rotate the support 10 about the z-axis as is required tostabilize the support during movement through the air either within ortowed behind an aircraft.

The specific embodiment also includes angular accelerometers which aresimilar in shape to the bars 41 and 42 but the shape is adjusted forzero quadruple moment. The linear accelerometers are simple pendulousdevices with a single micro pivot acting as the flexural hinge.

FIG. 9 is a cut away view of components of the gravity gradiometer readyfor mounting in the housing 1 which in turn is to be mounted in theexternal platform 2.

The transducers 71 measure the angle of displacement of the bars 41 and42 and control circuitry (not shown) is configured to measure thedifference between them. In this embodiment, the transducers 71 areconstant charge capacitors, which will be described in more detail withreference to FIG. 22.

FIGS. 10 to 15 show a second embodiment in which like parts indicatelike components to those previously described.

In the second embodiment the first mount 10 has cut-outs 80 whicheffectively form slots for receiving lugs (not shown) which areconnected to the mount 10 in the cut-outs 80 and also to the secondmount 20 shown in FIGS. 15 and 16. In this embodiment the lugs areseparate components so that they can be made smaller, and more easily,made than being cut with the second mount section 20.

In FIG. 10 a cut 87 is made to define the part 18 a of the hub 18. Thecut 87 then extends radially inwardly at 88 and then around centralsection 18 c as shown by cut 101. The cut 101 then enters into thecentral section 18 c along cut lines 18 d and 18 e to define a core 18f. The core 18 f is connected to the central section 18 c by theflexural web 31 which is an uncut part between the cut lines 18 e and 18d. The part 10 a therefore forms a primary mount portion of the mount 10which is separated from a secondary mount portion 10 a of the mount 10except for where the portion 18 a joins the portion 10 a by the flexuralweb 31. The part 18 a effectively forms an axle to allow for rotation ofthe part 18 a relative to the part 10 a in the z direction about theflexure web 31.

As is shown in FIG. 11, the cut line 88 tapers outwardly from the upperend shown in FIG. 11 to the lower end and the core 18 c tapers outwardlyin corresponding shape.

As is apparent from FIGS. 10, 12 and 13, the first mount 10 is octagonalin shape rather than round, as in the previous embodiment.

FIG. 14 shows a component of the second mount 20 for mounting in thefirst mount 10. As is best shown in FIGS. 14 and 15, the second mount 20has cut-outs 120 which register with the cut-outs 80 for receiving lugs(not shown). The lugs can bolt to the second mount 20 by bolts whichpass through the lugs and into bolt holes 121. The lugs (not shown) aremounted to the mount 20 before the mount 20 is secured to the firstmount 10.

In this embodiment, top wall 24 is provided with a central hole 137 andtwo attachment holes 138 a. Three smaller holes 139 a are provided tofacilitate pushing of the first housing portion 45 off the part 18 a ifdisassembly is required. When the second mount 20 is located within thefirst mount 10, the upper part of central section 18 c projects throughthe hole 137, as best shown in FIG. 13. The mount 20 can then beconnected to the mount 10 by fasteners which pass through the holes 138and engage in holes 139 b (see FIG. 10) in the part 18 a.

Thus, when the first housing portion 45 and its associated bar 41 isconnected to the rim 75 of the first mount 10 and the second housingportion 47 is connected to the base 12, flexure web 31 allows movementof the housing portions 45 and 47 about the z-axis.

Thus, when the second mount 20 is fixed to the part 18 a, the secondmount 20 can pivot with the first portion 10 a of the first mount 10about a z-axis defined by the flexure web 31 whilst the second portionformed by the part 18 a remains stationary.

FIG. 16 shows main body 61 of the housing 1 and connectors 69 with thehemispherical ends removed.

FIG. 17 is a plan view of the first housing portion 45 according to astill further embodiment of the invention. As is apparent from FIG. 17,the first housing portion 45 is circular rather than octagonal, as isthe case with the embodiment of FIG. 6.

The first housing portion 45 supports bar 41 in the same manner asdescribed via flexure web 59 which is located at the centre of mass ofthe bar 41. The bar 41 is of chevron shape, although the chevron shapeis slightly different to that in the earlier embodiments and has a morerounded edge 41 e opposite flexure web 59 and a trough-shaped wallsection 41 f, 41 g and 41 h adjacent the flexure web 59. The ends of thebar 41 have screw-threaded bores 300 which receive screw-threadedmembers 301 which may be in the form of plugs such as grub screws or thelike. The bores 300 register with holes 302 in the peripheral wall 52 aof the first housing portion 45. The holes 302 enable access to theplugs 301 by a screwdriver or other tool so that the plugs 301 can bescrewed into and out of the bore 300 to adjust their position in thebore to balance the mass 41 so the centre of gravity is at the flexureweb 59.

As drawn in FIG. 17, the bores 300 are a 45° angle to the horizontal andvertical. Thus, the two bores (302 shown in FIG. 17) are at right angleswith respect to one another.

FIG. 17 also shows openings 305 for receiving a portion of thetransducers 71 for monitoring the movement of the bar 41 and producingsignals in response to the movement. Typically, each transducer 71 is inthe form of a constant charge capacitor. One capacitor plate typicallyis mounted to the bar 41 and another capacitor plate is stationaryrelative to the bar 41 so that a gap is defined between the capacitorplates. Movement of the bar changes the gap which in turn changes avoltage across the constant charge capacitor.

FIG. 18 is a more detailed view of part of the housing portion of FIG.17 showing the openings 305. As can be seen from FIG. 18, the openings305 have shoulders 401 which form grooves 402.

FIG. 19( a) to (f) show portions of the constant charge capacitortransducers 71. The transducer shown in FIG. 19( a) comprises twoelectrodes. A first electrode is in this embodiment provided by asurface of the sensor bars 41 or 42, which are at ground potential, anda second electrode is shown in FIG. 19( a) (plate 408 a).

FIG. 19( b) shows the second capacitor electrode which comprises twoseparate capacitor elements 408 b and 407 b which are not in electricalcontact. Again, the first electrode is provided by the sensor bars 41 or42, which are at ground potential. The capacitor element 408 b surroundsthe capacitor element 407 b. This arrangement is used for generating a“virtual capacitor”, which will be described below with reference toFIG. 22.

FIGS. 19( c) and (d) show alternatives to the embodiment shown in FIG.19( b) and the shown second electrodes comprise adjacent elements 408 c,407 c and 408 d and 407 d respectively.

FIGS. 19( e) and (f) show capacitor elements according to furtherembodiments of the present invention. The second electrode comprisesthree capacitor elements 408 e, 407 e, 407 f and 408 f, 407 g and 407 h,respectively, and this arrangement is also used for generating a“virtual capacitor which will be described below.

It will be appreciated, that in variation of this embodiment thecapacitor plates may have any other suitable cross-sectional shape.

As an example, FIG. 20 shows the location of the capacitor elements 407b and 408 b in the opening 305 and opposite a corresponding secondcapacitor plate 411. In this embodiment the capacitor elements 407 b and408 b are provided in the form of metallic foils that are positioned oninsulating body 409. The plate 411 is metallic and positioned on the bar41. In this embodiment plate 411 provides one capacitor element thatopposes capacitor elements 407 b and 408 b. In this case the bar 41 maybe of relatively low electrical conductivity or may be electricallyinsulating.

If bar 41 is provided in the form of a metallic material of sufficientlyhigh electrical conductivity, the bar 41 itself may also provide acapacitor element and a portion of the bar 41 may directly oppose thecapacitor elements 407 b and 408 b without the plate 411, as discussedabove in the context of FIG. 17.

FIG. 21 is a diagram of the bars 41 and 42 showing them in their “inuse” configuration. The transducers which are located in the openings305 are shown by reference numbers 71 a to 71 e.

As will be apparent from FIG. 21, four transducers 71 are arrangedadjacent the ends of the bar 41. The second housing portion 47 also hasfour transducers arranged adjacent the bar 42. Thus, eight transducers71 are provided in the gradiometer.

Referring now to FIGS. 22 and 23 transducer circuitry 360 is nowdescribed. Each of the transducers 71 a to 71 e is a constant chargecapacitor and comprises a first capacitor electrode. Each of thetransducers 71 a to 71 e has a second capacitor electrode that ispositioned opposite a respective first capacitor electrode and fixed inposition relative to the housing portions. The first capacitor electrodeis in this embodiment provided by a surface the sensor bars 41 or 42.For example, each transducer 71 a-71 e may have a second electrode ofthe type as shown in FIG. 19.

Oscillating movement of the sensor masses 41 and 42 results in amovement of the first capacitor electrodes (surfaces of the sensor bars41 or 42) relative to the second capacitor electrodes. That movementchanges the gaps between respective first and second capacitorelectrodes and results in a voltage change across the constant chargecapacitor transducers 71 a to 71 e.

If the transducers are of the type as shown in FIG. 19( b) to 20(d),then separate component transducers are formed between the firstelectrode and each capacitor element of the second electrode, such as407 b and 408 b. In this case FIG. 22 shows the transducer circuitry forthe component transducers formed between the first plate and one of thetwo elements and an analogous circuitry (labeled accordingly) is usedfor the component transducers formed between the first electrode and theother capacitor elements.

If the transducers are of the type as shown in FIGS. 19( e) and 19(f),then separate component transducers are formed between the firstelectrode and each of the three capacitor elements, such as 408 e, 408 eand 407 f. In this case. FIG. 22 shows the transducer circuitry for thecomponent transducers formed between the first electrode and one of thethree elements and two analogous circuitries (labeled accordingly) areused for the component transducers formed between the first plate andthe other capacitor elements.

Each constant charge capacitor component transducer 71 a to 71 e has aseparate bias voltage by a respective bias voltage source V_(Bαβγ)applied to it. FIG. 22 shows component transducer 71 a to 71 e with oneof the capacitor elements being connected to ground potential. Asdiscussed above, these capacitor elements are surfaces of the sensorbars 41 and 42, which are in this embodiment electrically conductive andconnected to ground potential. The polarities of the voltages providedby the bias voltage sources 361 a to 361 e and the electricalinterconnections between the constant charge capacitor componenttransducers 71 a to 71 e are chosen so that the electrical signalsgenerated by all transducers are combined with the same polarity if thesensor masses 41 and 42 oscillate in opposite directions. Suchoscillation in opposite directions typically is generated by a gravitygradient. If the sensor masses 41 and 42 move in the same direction, onehalf of the electrical signals generated by the constant chargecapacitors component transducers 71 a to 71 e has one polarity and theother half has an opposite polarity. Consequently, in this case, theelectrical signals typically cancel each other. Such movement in thesame direction may for example be generated by a sudden movement of theaircraft in which the gravity gradiometer is positioned and consequentlythe transducer circuitry 360 illustrated in FIG. 22 reduces the effectof such sudden movements and the effect of a number of other externalforces or external angular accelerations that are not related to thegravity gradient.

The combined electrical signal is directed to a low noise amplifierwhich will be described in the context of FIG. 23.

The transducer circuitry 360 shown in FIG. 22 also comprises lockingcapacitors C_(Sαβγ) which are arranged so that the applied bias voltagesV_(Bαβγ) cannot reach the lower noise amplifier. The locking capacitors362 a to 362 e typically have a capacitance that is larger than 10times, or even larger than 100 times that of the respective constantcharge capacitor component transducers 71 a to 71 e.

Further, the transducer circuitry 360 comprises resistors R_(Bαβγ) 363 ato 363 e. These resistors typically have a very high resistance, such as1 GΩ or more, and are arranged for substantially preventing flow ofcharges and thereby providing for the component transducers 71 a to 71 eto operate as constant charge capacitors.

The bias voltages applied to the constant charge capacitors generateelectrostatic forces. Consequently, each transducer 71 a to 71 e canalso function as an actuator.

If the transducers 71 are of the type as shown in FIG. 19( a), then thecircuitry 360 shown in FIG. 22 is sufficient. However in a specificembodiment of the present invention the transducers are of the type asshown in FIGS. 19( b) to 19(d) and comprise two component transducers.In this case two circuitries 360 are used, one for the componenttransducers formed between the first electrodes and one of the capacitorelements, and the other for the component transducers formed between thefirst electrodes and the other capacitor elements. This is schematicallyindicated in FIG. 25. A first circuitry 360 is used for measurementpurposes (differential mode, “DM”) and a second circuitry 360 is used toprovide feedback for external rotational motion correction (common mode,“CM”), which will be described below with reference to FIGS. 28 and 29.

Alternatively, the circuitries 360 may also be connected so that“virtual capacitors” are formed. This will be described below in moredetail and is schematically indicated in FIG. 24.

In another specific embodiment of the present invention the transducersare of the type as shown in FIGS. 19( e) or 19(f) and comprise threecomponent transducers. In this case three circuitries 360 are used. Thisis schematically indicated in FIG. 26. In this embodiment twocircuitries 360 are used for measurement purposes and arranged so that“virtual capacitors” are formed. A third circuitry 360 is used toprovide feedback for external rotational motion correction.

The following will describe how relative mechanical properties of thesensor masses 41 and 42 can be tuned. The resonance frequencies of thesensor masses 41 and 42 depend on the square of the electrostatic forcesand therefore the square of the applied bias voltage. For example, theresonance frequencies may be tuned using a mechanical test set up inwhich external forces are applied to the sensor masses 41 and 42. If theresonance frequencies are not identical, the bias voltages can beadjusted until the resonance frequencies are identical.

The sensitivities of the transducer capacitors for sensing the movementof the sensor masses is linearly dependent on the electrostatic forcesand thereby linearly dependent on the applied bias voltages.Consequently, it is possible to tune both the resonance frequencies andthe sensitivities of the transducers

FIG. 23 shows a schematic circuit diagram of a low noise amplifieraccording to a specific embodiment of the present invention. The lownoise amplifier circuitry 366 is used to amplify the electrical signalgenerated by the transducer circuit 360 and to provide active feedbackto control properties of the transducers and sensor masses 41 and 42.

The amplifier circuit 366 simulates an impedance Z_(L) and an ohmiccomponent of Z_(L) provides active damping of resonant electricalsignals generated by the constant charge capacitor component transducers71 a to 71 e described above. The active damping reduces the Q-factor ofthe resonance and thereby increases the bandwidth within which theresonance can be generated. That electrical damping results inmechanical damping by generating electrostatic damping forces at theconstant charge capacitor component transducers 71 a-71 e. Typically,the active damping is adjusted so that the gravity gradiometer has abandwidth of the order of 1 Hz and the Q-factor of the active damping isclose to 0.5.

The impedance Z_(L) also has an imaginary component, which is dependenton a simulated capacitance C_(L) in parallel with the simulated resistorR_(L). The imaginary component actively controls the resonance frequencyof the sensor masses 41 and 42 via the constant charge capacitortransducers 71 a-71 e by simulating a change of the “stiffness” of thepivotal coupling of the sensor masses 41 and 42 and thereby fine-tunesthe resonance frequency of the sensor masses 41 and 42. As describedabove, the transducer circuit 360 is arranged so that resonantoscillations in which the sensor masses 41 and 42 oscillate in oppositedirections result in an additive electrical signal. The simulatedcapacitance C_(L) of the simulated impedance Z_(L) allows fine tuning ofthe resonance and thereby further helps distinguishing that resonanceoscillation from other common mode oscillations in which the sensormasses 41 and 42 oscillate in the same direction.

In this embodiment the amplifier circuit 366 provides “cold damping”,which introduces very little thermal noise. Passive damping, such asdamping using a conventional resistor, is avoided as this would resultin thermal noise.

As described above, the constant charge component capacitors 71 a-71 emay combine sensing and actuator functions. The amplifier circuit 366provides an active feedback loop between sensing and actuator functionsand provides electronic feedback control of mechanical properties of thesensor masses 41 and 42.

The amplifier circuit 366 comprises an input 368 and an output 369.Further, the amplifier circuit 366 comprises a low-noise j-FETdifferential amplifier 370 and impedances Z1, Z2 and Z3. The low noiseamplifier 370 has two input terminals 371 and 372 and the impedance Z₁is connected between the output terminal 369 and the low noise amplifierinput 371. The impedance Z₂ is connected between the output terminal 369and the low noise amplifier input 372. The impedance Z₃ is connectedbetween the terminal 372 and a common ground terminal 373.

The amplifier circuit 366 simulates the impedance Z_(L) with

$\begin{matrix}{Z_{L} \approx {- {\frac{Z_{1}Z_{3}}{Z_{2}}.}}} & \left( {{eq}.\mspace{14mu} 1} \right)\end{matrix}$

The amplifier 370 has noise matched resistance

$R_{opt} = {\sqrt{\frac{S_{V}}{S_{i}}}.}$

The term S_(v) is the spectral density of amplifier's voltage noise andthe term S_(i) is the spectral density of amplifier's current noise. Inthis embodiment the amplifiers noise matched resistance is a few 1 MΩ.

Further, the amplifier 370 has a noise temperature

$T_{opt} = \frac{\sqrt{S_{V}S_{i}}}{2k_{B}}$

(k_(B): Bolzman constant) of less than 1K.

The noise density S_(Γ) of the gradient error produced by thermal noisenear resonance is given by

$\begin{matrix}{S_{\Gamma} = \frac{4k_{B}T_{opt}2\pi \; f_{0}}{m\; \lambda^{2}Q_{act}}} & \left( {{eq}.\mspace{14mu} 2} \right)\end{matrix}$

where λ is the radius of the gyration of the sensor masses 41 and 42 andQ_(act) the effective Q-factor associated with the active damping, M isthe mass of the senor masses 41 and 42 and f_(o) is the resonancefrequency. The noise density S_(Γ) is dependent on the noise of theamplifier and not on the physical temperature of the amplifier circuit,which allows “cold damping” and control of other mechanical propertieswithout introducing significant thermal noise at normal operationtemperatures such as at room temperature.

The component transducers 71 a, 71 b, 71 g and 71 h are also used toform angular accelerometers for measuring the angular movement of themounting 5 so that feedback signals can be provided to compensate forthat angular movement.

FIG. 27 shows an actuator for receiving the control signals to adjustthe mounting in response to angular movement of the mounting 5.

The actuator shown in FIG. 27 is also schematically shown in FIG. 8 byreference to numerals 53 and 54. The actuators are the same and FIG. 28will be described with reference to the actuator 54.

The actuator 54 comprises in this embodiment a permanent NdFeB magnet410, a soft iron core 411, a non-magnetic spacer 412 (aluminum, delrin),mumetal or permalloy housing 413, a voice coil assembly 414, a hollowrod 428 and a tube 430 that forms part of the housing 413 and in whichthe hollow rod 428 is rotatably mounted.

The voice coil assembly 414 is mounted onto rod 430 and the permanentmagnet 410 and the soft iron core 411 are provided with internal boresthrough which the rod 430 penetrates so that the rod 430 with voice coilassembly 414 can move axially relative to the iron core 311 and themagnet 410. Electrical connections for the voice coil assembly 414 arefed through the hollow rod 430.

As described above, one or both of the bars 41 and 42 can also be usedas an angular accelerometer to provide a measure of angular movement ofthe mounting 5 so that appropriate feedback signals can be generated tocompensation for that movement by control of the actuators previouslydescribed.

FIGS. 28( a) and (b) show schematic plan and cross-sectional view of thegravity gradiometer 1. As indicated previously, the gravity gradiometer1 comprises a housing 2 that is rotated by an external mounting about az-axis. The external mounting comprises an inner stage 500 and anintermediate stage 502 and an outer stage 504. The housing 2 is mountedso that it is rotated with the inner stage 500 by z-drive 508 withbearings. The z-drive provides continuous rotation at a very stablespeed. The rotational frequency is in this embodiment selectable between0 and 20 Hz. The intermediate stage 502 including the inner stage 500 isrotable about the x-axis by x-drive 510, which includes bearings and theouter stage 504 is rotable with the intermediate stage 502 about they-axis by y-axis drive 512 which also include suitable bearings. Theouter stage with y-axis drive is mounted on springs 516 in a supportframe 518.

The external mount 3 includes an IMU (inertial measurement unit), whichcontains gyroscopes, accelerometers, GPS receivers and a computer. TheIMU is not shown in FIG. 28( a) or (b). The IMU measures rotation aboutthe x-, y- and z-axis and is coupled to drives in a feedback loop. Thiswill be described below in more detail with reference to FIG. 29.

The external mounting is arranged to gyro-stabilize the housing 2 aboutthe x-,y- and z-axis with a gain factor of approximately 100 DC and abandwidth of 20 Hz. This is achieved using the above-described 3-axis“gimbal” bearing arrangement with direct drive torque motors (508, 510and 512). In this embodiment, fine-tuning of the motor drive forcorrection of rotation about the z-axis is achieved using the “commonmode” signal provided by respective transducer components positionedwithin the housing 2.

FIG. 29 shows a block diagram 600 that illustrates how the common modesignal, generated within the housing 2 (“internal platform”), is usedfor rotational z-axis correction of the external support structure(“external platform”).

Blocks 602 and 604, labelled “response to motion” and “response toforce” respectively, both represent the gimbal structure of the supportstructure 3. Each gimbal consists of three main components, namely aframe, a part supported by the frame via a bearing and an actuator whichapplies a torque (force) to this part. Each gimbal has two independentinputs, namely motion applied to the frame and a force applied directlyto the part suspended by the frame. It has only one output, namely theangular position of the supported part and this responds differently tothe two inputs.

Feedback force F_(e) counteracts an external disturbance Z. This may beexpressed by the following equation

X _(e) =H _(f) F _(e) +H _(z) Z   (eq. 3)

where H_(f) and H_(z) are constants.

Equation 3 may be written as

X _(e) =H _(f)(F _(e) +K _(e) Z)   (eq. 4)

where K_(e)=H_(z)/H_(f).

An external motion, such as a motion of an aircraft in which the gravitygradiometer 1 is positioned, produces an equivalent force K_(e) Z, whichis counteracted by F_(e) generated by the actuator 610. In FIG. 29 block602 “Response to motion” represents K_(e) and block 604 “Response toforce” represents H_(e). The sensor 606 for the external platform is theIMU, which contains gyroscopes, accelerometers, GPS receivers and acomputer. This provides a signal (usually digital) which measures theangular position and angular rate of the supported part of the innermostgimbal. This signal is used in the controller 608 (also usually digital)to implement the feedback.

The internal platform may be represented in an analogous manner whereblocks 612 and 614 labelled “response to motion” and “response to force”respectively, both represent the z-axis gimbal structure within thehousing 2. The transducer sensors 71 and the actuator 54 have beendescribed above.

In the above-described embodiment the gravity gradiometer 1 is arrangedso that rotation about the z-axis is controlled to a fixed uniformrotation speed. The input signal for controlling the motion is providedby the IMU 606 and directed to the controller 608. However, the IMU 606may only have limited accuracy at the higher frequencies and to improvethe z-axis rotational correction further, an angular accelerationderived from the above-described “Common Mode” signal from the internaltransducers 71 is used for fine-tuning. This same signal is also usedinside the internal platform in a feedback loop to stabilize theinstrument against applied angular acceleration (via actuator 54). Thespecification for this internal feedback system is stringent and to easethis requirement, some of the burden is transferred to the externalplatform in that manner.

In a variation of the above-described embodiment the IMU may also beused in a feed-forward configuration.

FIG. 30 shows a block diagram 650 that illustrates stabilization (norotation) about the x-and y-axis, which is performed exclusively by theexternal platform. All elements of FIG. 30 were already described aboveand function in an analogous manner to inhibit rotation about the x- andy-axes.

Although the invention has been described with reference to particularexamples, it will be appreciated by those skilled in the art that theinvention may be embodied in many other forms. For example, thetransducers may not necessarily be provided in the form of constantcharge capacitors, but may be provided in the form of any other suitabletype of capacitor including those that do not allow simulation of avirtual capacitor. Further, it is to be appreciated that the amplifiercircuitry 366 shown in FIG. 24 is only one embodiment and a variety ofvariations from the described embodiment are possible.

In addition, the gravity gradiometer may be arranged for measuring othercomponents of the gravity gradient, in which case the gravitygradiometer would not be arranged for operation in the describedorientation. For example, the gravity gradiometer may be arranged tomeasure the Γ_(yz) and (Γ_(zz)-Γ_(yy)) or Γ_(xz) and (Γ_(zz)-Γ_(yy)) ofthe gravity gradient.

The reference that is being made to documents WO 90/07131 andPCT/AU2006/001269 does not constitute an admission that these documentsform a part of the common general knowledge in Australia or in any othercountry.

In the claims which follow and in the preceding description of theinvention, except where the context requires otherwise due to expresslanguage or necessary implication, the word “comprise” or variationssuch as “comprises” or “comprising” is used in an inclusive sense, i.e.to specify the presence of the stated features but not to preclude thepresence or addition of further features in various embodiments of theinvention.

1. A gravity gradiometer for measuring components of the gravitygradient tensor, the gravity gradiometer comprising: a housing; agimballed support structure for supporting the housing and rotating thehousing about an axis, the support structure comprising an actuator; atleast one sensor mass being positioned in the housing and being arrangedfor movement in response to a gravity gradient; a pivotal couplingenabling the movement of the at least one sensor mass about an axis andsuspending the at least one mass in the housing; a transducer arrangedso that the movement of the at least one sensor mass relative to aportion of the transducer generates a transducer signal; and anelectrical feedback loop arranged to direct an electrical signal to theactuator of the external support structure, the electrical signal beingassociated with the transducer signal; wherein the gravity gradiometeris arranged so that an influence of an external angular acceleration onthe rotation of the housing is reduced by the actuator when thetransducer generates a transducer signal effected by the externalangular acceleration on the gravity gradiometer.
 2. The gravitygradiometer of claim 1 being arranged for movement over a ground planethat defines an x-y plane of an x,y,z Cartesian coordination system andwherein the support structure is arranged so that in use the housing isrotated about an axis oriented substantially along the z-direction ofthe x,y,z Cartesian coordination system.
 3. The gravity gradiometer ofclaim 1 wherein the gimballed support structure is triaxial andcomprises three substantially orthogonal axes.
 4. The gravitygradiometer of claim 3 wherein the gimballed support structure comprisesat least two additional actuators arranged for rotating the housingabout each of the three substantially orthogonal axes.
 5. The gravitygradiometer of claim 4 wherein the gimballed support structure comprisesgyroscopes that detect rotation about respective axes and that arecoupled to respective actuators in a manner such that the actuatorscorrect for rotation generated by the external angular acceleration. 6.The gravity gradiometer of claim 1 wherein the electrical feedback loopis arranged so that rotation of the housing about the z-axis is finetuned and the influence of the external angular acceleration on therotation about the z-axis is substantially avoided.
 7. The gravitygradiometer of claim 1 wherein the transducer comprises a capacitor. 8.The gravity gradiometer of claim 1 wherein the pivotal couplingcomprises a flexure web for connecting the at least one sensor mass in ahousing for movement in response to the gravity gradient.
 9. The gravitygradiometer of claim 1 comprising a pair of transversally arrangedsensor masses for movement in response to the gravity gradient.
 10. Thegravity gradiometer of claim 9 wherein the transducer is one of aplurality of transducers and each sensor mass is associated with two ormore transducers arranged so that movement of the sensor massesgenerates transducer signals that are associated with the electricalsignal used in the electrical feedback loop.